The present invention relates to an ultrasonic wire bonding apparatus and method for generating ultrasonic vibrations, guiding a wire, and bonding the wire on a specified surface with a high degree of precision. As one example, the ultrasonic wire bonding apparatus and method can be used in coupling a printed circuit board and a semiconductor chip with a wire. The ultrasonic wire bonding apparatus and method can also be constituted such that the wire bonding method for bonding a wire by employing ball bonding techniques may be applied.
Ultrasonic wire bonding is usually employed in semiconductor manufacturing processes or the like. An ultrasonic wire bonding apparatus (tool) is used for attaching a tiny wire between a semiconductor chip and a package or a workpiece. Herein, the workpiece generally refers to an integrated circuit (IC) package or a printed circuit board.
An ultrasonic wire bonding apparatus is designed to bond a wire by vibrating the wire at a high frequency of about 60,000 Hz or more. In this ultrasonic wire bonding apparatus, the wire is vibrated while pressing the wire against the bonding surface. The wire oscillates so as to be displaced in a parallel direction to the horizontal plane of the bonding surface. As the wire oscillates against the bonding surface, plastic deformation of the wire and the bonding surface occurs. By this deformation, atoms on the wire and bonding surface are bonded, and thus a cold weld bond is formed. Bonding is achieved when the cold weld bond is formed.
The ultrasonic wire bonding apparatus is designed to bond one end of a wire to a semiconductor chip, guide the wire to a workpiece, and bond the other end of the wire to the workpiece. Afterwards, by cutting the wire, the above operation is repeated, and another connection operation is performed.
More specifically, as shown in FIG. 9, a capillary 102 is vertically fixed to the end of a horn (an ultrasonic horn) 101 for applying ultrasonic vibrations, and after pressing a lower end of a wire 106 passing through the capillary 102 against the bonding surface of a semiconductor chip (bare chip) 104 placed on a work table 103 with a small pressure and bonding by ultrasonic vibration, the wire 106 is let out slightly and pressed again with a small pressure against a bonding surface of a lead frame 105 supported on the work table 103, and bonded by ultrasonic vibration, and then the wire 106 is cut.
The ultrasonic wire bonding apparatus usually includes an actuator for generating ultrasonic waves, a horn for applying ultrasonic vibrations to transmit excitation ultrasonic waves, and a capillary for holding the wire. The capillary is supported at the lower end of the horn. An electrostrictive vibrator or magnetostrictive vibrator, which is a vibration actuator of electric drive system, is used for generating mechanical vibrations at a frequency in an ultrasonic range. The horn transmits the vibrations generated by the actuator to the capillary by compression waves. Usually, the horn is tapered at its lower end, and is used for amplifying vibrations in the capillary. The capillary holds the wire with the wire slightly projecting from the lower end of the capillary. Afterwards, the lower end of the capillary is pressed against the bonding surface. The capillary oscillates due to the vibrations from the horn, and hence the wire projecting from the lower end of the capillary is bonded to the bonding surface.
The horn, which is necessary for wire bonding, is installed in a device which supports the horn and determines its vertical position. The device is generally mounted on an x-y table which is capable of moving the position of the horn along the horizontal direction of the bonding surface, and constitutes the bonding apparatus. This x-y table slides longitudinally and laterally in order to accurately locate the wire bonding end at a desired position. The wire is vertically positioned by pivoting the horn at its support point. Accordingly, the lower end of the capillary moves vertically along an arc along the lengthwise direction of the capillary.
However, the conventional ultrasonic wire bonding apparatus has the following disadvantages.
One of the major disadvantages of the existing horn design is that a clearance is hardly provided between the workpiece (bonding surface) and the horn because the capillary length is short. As a result, the working area, that is, the bonding range is limited. This is because the horn hits the workpiece at the horn supporting position.
Use of a long horn in order to increase the working area has been considered. However, a long horn has two major disadvantages. A first disadvantage is that the position precision of bonding is lowered because the thermal expansion of the horn is large. To enhance the quality of bonding, a heating table for heating the workpiece is always required. The heat from the table raises the horn temperature, and thus expands the horn. The degree of thermal expansion is greater as the horn becomes longer. Expansion of the horn leads to a lower degree of precision with respect to the position of the bond. A second disadvantage of the long horn is an increase of mass along with a size increase. As the horn mass increases, greater power is needed for oscillating the horn. As the mass increases, greater energy is needed to move a specified distance.
Aside from these two disadvantages, deflection occurs in the horn causing vibration in the vertical direction of the horn, which may cause damage to workpieces such as semiconductor chips.
The other disadvantage due to lack of clearance between the workpiece and horn is the difficulty in bonding wire to a board having large components. In the surface mounting technology known as chip-on-board, a semiconductor chip is mounted directly on a printed circuit board. Since the printed circuit board contains numerous components at various levels of height, it is necessary to ensure that the horn has the proper clearance relative to the components in order to bond the semiconductor chip directly on the printed circuit board.
Another disadvantage of the existing horn design arises from the pivot of the horn used in moving the capillary vertically. The capillary must be perpendicular to the bonding surface for appropriate bonding. Such vertical positioning is required, but since the horn has the pivot, the capillary moves along an arc. Therefore, the pivot angle of the horn is limited to a very small angle. In such horn design, however, wire bonding on a surface of non-uniform height is not possible. That is, the capillary may not always keep be able to maintain a vertical position relative to the bonding surface. Furthermore, the pivoting device is positioned at the end of the horn. Depending on the position and size of the pivoting device, when the horn sufficiently reaches up to above the workpiece, the workpiece touches the pivoting device. This results in a decrease in the working space.